This patent relates to MEMS microphone incorporating a reset for a high-voltage high-impedance network for the microphone bias node allowing faster, more efficient testing of the MEMS microphone.
MEMS capacitive microphones operate utilizing conservation of charge. A high impedance network, usually consisting of two anti-parallel diodes is used to apply a fixed charge across two plates of a capacitor. A high impedance network on a sense node is necessary to create the charge conservation node. A high impedance switch network on a bias node is followed by a capacitor to ground which is large in comparison to the capacitance from the sense node to ground. This capacitor serves two purposes. First the capacitor creates an AC ground on the bias side of the sensor so that in the presence of an acoustic signal, the voltage on the sense node changes. Second, the capacitor along with the high impedance network creates a low-pass filter for noise generated by the biasing circuits.
Leakage current from the high-impedance bias node to ground can degrade the performance of the microphone. The leakage current from bias to ground lowers the impedance of the high impedance network and if large enough can compromise the noise filtering of the bias circuits, ultimately degrading the noise performance of the entire microphone. Similarly, leakage current from the bias node to the sense node flows into the sense node diodes giving rise to shot-noise which also degrades the noise performance of the entire microphone. Defects near these high impedance nodes which can be due to particles, surface contamination, or bulk material defects can give rise to these leakage currents which will affect the high impedance network. Furthermore these defects can be exacerbated through reliability and environmental stresses making early detection even more important to ensuring the quality of the microphone.
U.S. patent application Ser. No. 13/040,466 describes one of many implementations for realizing high-voltage high impedance circuits.